misfire and erratic tach signal - TCCoA Forums
 
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post #1 of 9 (permalink) Old 12-01-2009, 08:33 PM Thread Starter
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misfire and erratic tach signal

I recently bought a 95 sc 5 speed. I have an intermittent check engine light along with a tach signal that drops to zero under accel, and returns at cruise but doesn't seem to always read the correct rpm. It has a rough idle and misfires under low rpm load and runs well under boost but will occasionally fall flat on its face under boost. I tried replacing the cam position sensor with no change in performance. I cant check the code because I dont have an obd I reader.

thanks
-steve
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post #2 of 9 (permalink) Old 12-01-2009, 09:21 PM
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Have you checked your connections at I/C, blower, and intake? Almost sounds like a loose flange/nut and letting air in/boost out. I had a similar problem when I had to rebuild my 3.8. Looked at plugs, wires, firing order, etc., etc. Turns out I just forgot to tighten the collar nut on the blower top. Can't help you with the tach though, sorry.

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post #3 of 9 (permalink) Old 12-05-2009, 11:14 AM Thread Starter
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I posted these over on sccoa just seeing if anyone over here has some ideas too. well I bought a scanner and scanned the codes, this is what I came up with:

Immediate:
522 - vehicle transmission not in park during KOEO self test.
Its a five speed so I'm not sure how it could have that one.

Stored:

124 - Throttle position sensor - signal voltage is higher than expected
171 - heated exhaust gas oxygen sensor voltage signal not switching bank 1
172 - heated exhaust gas oxygen sensor voltage signal indicates lean bank 1
181 - heated exhaust gas oxygen sensor voltage signal not switching : Indicates lean during part throttle engine operation bank 1
185 - mass air flow sensor - signal voltage is lower than expected
189 - heated exhaust gas oxygen sensor voltage signal not switching : Indicates lean during part throttle engine operation bank 2
214 - Cylinder Identification Sensor - circuit failure

Engine Running:

538 - Insufficient RPM change during dynamic response test (engine run self test) or invalid cylinder balance test - throttle position movement, or invalid cylinder balance test - cylinder identification problems

any suggestions would help
thanks,
-steve
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post #4 of 9 (permalink) Old 12-05-2009, 12:04 PM
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Replace the ignition module. It is a fairly common problem on the SCs, and the tach signal cutting in and out is a dead giveaway. The O2 sensor codes are all probably due to the misfires, which are probably caused by the failing DIS module. You may also have a TPS issue since the DIS module failing couldn't throw a TPS code. The MAF code might be related to the TPS, or it might be a vacuum leak or a dirty MAF. If it were my car, I would replace the DIS module and the TPS and then if there are still issues look for vacuum leaks. That is one thing about the SCs is that a little vacuum leak can cause all kinds of weird drivability problems, and they are a PITA to diagnose, but on the bright side they are usually very cheap to fix once you do find the problem.

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post #5 of 9 (permalink) Old 12-05-2009, 01:32 PM Thread Starter
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thanks for the help, I'll be getting those parts and clean my maf.
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post #6 of 9 (permalink) Old 12-12-2009, 10:28 PM Thread Starter
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just an update on this deal. I cleaned the maf, replaced the tps, and the ignition control module which was corroded. I also installed a screamin demon coil pack and live wires. All that and the alternator took a crap two days ago so I replaced that. the car runs perfect now with the exception of a bank 1 o2 code and an exhaust leak on that side which may be causing the code. the tach still drops to zero under accel. and seems to read incorrectly at cruise. anyone know what the tach gets its signal from so I can find a wiring diagram and test the circuit?
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post #7 of 9 (permalink) Old 12-13-2009, 06:30 AM
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Originally Posted by slidebird91 View Post
just an update on this deal. I cleaned the maf, replaced the tps, and the ignition control module which was corroded. I also installed a screamin demon coil pack and live wires. All that and the alternator took a crap two days ago so I replaced that. the car runs perfect now with the exception of a bank 1 o2 code and an exhaust leak on that side which may be causing the code. the tach still drops to zero under accel. and seems to read incorrectly at cruise. anyone know what the tach gets its signal from so I can find a wiring diagram and test the circuit?
First, be careful with that screamin' demon coil pack. A lot of guys have had problems with it and the wires. Most guys will just run FRPP or Taylor Pro wires on their SC's.


Next, the problem you're seeing could be caused by the crank sensor. When it fails, the car will simply refuse to start. It controls injector timing, and ignition. BUT, if the dampener for the crank is damaged/needs replaced the signal to the crank sensor can be going in and out. While the motor is running, look straight down the front of the crank pulley to see if there is any wobble. Or, you could simply have a intermittent sensor. My '94 would run like ass for a day or so, and then all of a sudden would run perfect.


I would check all your wiring as well, In particular the main harness piece that runs from the coil pack, under the blower to the crank sensor, to the dis module, and around the block to the passenger side firewall. On my SC I found a few sections with damaged wires. The crank sensor connector was also damaged and had grime all over it.

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post #8 of 9 (permalink) Old 12-13-2009, 09:24 AM Thread Starter
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I checked the damper as instructed before and it seems to be perfectly fine with no wobble. Ill have a closer look at the wiring. it would really help if I could find a wiring diagram
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post #9 of 9 (permalink) Old 12-13-2009, 10:02 AM
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The tach signal wire is tapped into the wire that connects the ignition control module to the PCM. It is an orange/white wire coming out of pin 2 on the ICM and pin 4 on the ECM. The tach signal wire is white/pink and is tapped into that wire.

-91 Cougar LS, coming soon, complete overhaul with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
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